open-nomad/nomad/blocked_evals.go

package nomad

import (
	"sync"
	"time"

	"github.com/armon/go-metrics"
	"github.com/hashicorp/nomad/nomad/structs"
)

const (
	// unblockBuffer is the buffer size for the unblock channel. The buffer
	// should be large to ensure that the FSM doesn't block when calling Unblock
	// as this would apply back-pressure on Raft.
	unblockBuffer = 8096
)

// BlockedEvals is used to track evaluations that shouldn't be queued until a
// certain class of nodes becomes available. An evaluation is put into the
// blocked state when it is run through the scheduler and produced failed
// allocations. It is unblocked when the capacity of a node that could run the
// failed allocation becomes available.
type BlockedEvals struct {
	evalBroker *EvalBroker
	enabled    bool
	stats      *BlockedStats
	l          sync.RWMutex

	// captured is the set of evaluations that are captured by computed node
	// classes.
	captured map[string]*structs.Evaluation

	// escaped is the set of evaluations that have escaped computed node
	// classes.
	escaped map[string]*structs.Evaluation

	// unblockCh is used to buffer unblocking of evaluations.
	capacityChangeCh chan string

	// jobs is the map of blocked job and is used to ensure that only one
	// blocked eval exists for each job.
	jobs map[string]struct{}

	// duplicates is the set of evaluations for jobs that had pre-existing
	// blocked evaluations. These should be marked as cancelled since only one
	// blocked eval is neeeded bper job.
	duplicates []*structs.Evaluation

	// duplicateCh is used to signal that a duplicate eval was added to the
	// duplicate set. It can be used to unblock waiting callers looking for
	// duplicates.
	duplicateCh chan struct{}

	// stopCh is used to stop any created goroutines.
	stopCh chan struct{}
}

// BlockedStats returns all the stats about the blocked eval tracker.
type BlockedStats struct {
	// TotalEscaped is the total number of blocked evaluations that have escaped
	// computed node classes.
	TotalEscaped int

	// TotalBlocked is the total number of blocked evaluations.
	TotalBlocked int
}

// NewBlockedEvals creates a new blocked eval tracker that will enqueue
// unblocked evals into the passed broker.
func NewBlockedEvals(evalBroker *EvalBroker) *BlockedEvals {
	return &BlockedEvals{
		evalBroker:       evalBroker,
		captured:         make(map[string]*structs.Evaluation),
		escaped:          make(map[string]*structs.Evaluation),
		jobs:             make(map[string]struct{}),
		capacityChangeCh: make(chan string, unblockBuffer),
		duplicateCh:      make(chan struct{}, 1),
		stopCh:           make(chan struct{}),
		stats:            new(BlockedStats),
	}
}

// Enabled is used to check if the broker is enabled.
func (b *BlockedEvals) Enabled() bool {
	b.l.RLock()
	defer b.l.RUnlock()
	return b.enabled
}

// SetEnabled is used to control if the broker is enabled. The broker
// should only be enabled on the active leader.
func (b *BlockedEvals) SetEnabled(enabled bool) {
	b.l.Lock()
	if b.enabled == enabled {
		// No-op
		return
	} else if enabled {
		go b.watchCapacity()
	} else {
		close(b.stopCh)
	}
	b.enabled = enabled
	b.l.Unlock()
	if !enabled {
		b.Flush()
	}
}

// Block tracks the passed evaluation and enqueues it into the eval broker when
// a suitable node calls unblock.
func (b *BlockedEvals) Block(eval *structs.Evaluation) {
	b.l.Lock()
	defer b.l.Unlock()

	// Do nothing if not enabled
	if !b.enabled {
		return
	}

	// Check if the job already has a blocked evaluation. If it does add it to
	// the list of duplicates. We omly ever want one blocked evaluation per job,
	// otherwise we would create unnecessary work for the scheduler as multiple
	// evals for the same job would be run, all producing the same outcome.
	if _, existing := b.jobs[eval.JobID]; existing {
		b.duplicates = append(b.duplicates, eval)

		// Unblock any waiter.
		select {
		case b.duplicateCh <- struct{}{}:
		default:
		}

		return
	}

	// Mark the job as tracked.
	b.stats.TotalBlocked++
	b.jobs[eval.JobID] = struct{}{}

	// If the eval has escaped, meaning computed node classes could not capture
	// the constraints of the job, we store the eval separately as we have to
	// unblock it whenever node capacity changes. This is because we don't know
	// what node class is feasible for the jobs constraints.
	if eval.EscapedComputedClass {
		b.escaped[eval.ID] = eval
		b.stats.TotalEscaped++
		return
	}

	// Add the eval to the set of blocked evals whose jobs constraints are
	// captured by computed node class.
	b.captured[eval.ID] = eval
}

// Unblock causes any evaluation that could potentially make progress on a
// capacity change on the passed computed node class to be enqueued into the
// eval broker.
func (b *BlockedEvals) Unblock(computedClass string) {
	// Do nothing if not enabled
	if !b.enabled {
		return
	}

	b.capacityChangeCh <- computedClass
}

// watchCapacity is a long lived function that watches for capacity changes in
// nodes and unblocks the correct set of evals.
func (b *BlockedEvals) watchCapacity() {
	for {
		select {
		case <-b.stopCh:
			return
		case computedClass := <-b.capacityChangeCh:
			b.unblock(computedClass)
		}
	}
}

// unblock unblocks all blocked evals that could run on the passed computed node
// class.
func (b *BlockedEvals) unblock(computedClass string) {
	b.l.Lock()
	defer b.l.Unlock()

	// Protect against the case of a flush.
	if !b.enabled {
		return
	}

	// Every eval that has escaped computed node class has to be unblocked
	// because any node could potentially be feasible.
	var unblocked []*structs.Evaluation
	if l := len(b.escaped); l != 0 {
		unblocked = make([]*structs.Evaluation, 0, l)
		for id, eval := range b.escaped {
			unblocked = append(unblocked, eval)
			delete(b.escaped, id)
			delete(b.jobs, eval.JobID)
		}
	}

	// We unblock any eval that is explicitly eligible for the computed class
	// and also any eval that is not eligible or uneligible. This signifies that
	// when the evaluation was originally run through the scheduler, that it
	// never saw a node with the given computed class and thus needs to be
	// unblocked for correctness.
	for id, eval := range b.captured {
		if elig, ok := eval.ClassEligibility[computedClass]; ok && !elig {
			// Can skip because the eval has explicitly marked the node class
			// as ineligible.
			continue
		}

		// The computed node class has never been seen by the eval so we unblock
		// it.
		unblocked = append(unblocked, eval)
		delete(b.jobs, eval.JobID)
		delete(b.captured, id)
	}

	if l := len(unblocked); l != 0 {
		// Update the counters
		b.stats.TotalEscaped = 0
		b.stats.TotalBlocked -= l

		// Enqueue all the unblocked evals into the broker.
		b.evalBroker.EnqueueAll(unblocked)
	}
}

// GetDuplicates returns all the duplicate evaluations and blocks until the
// passed timeout.
func (b *BlockedEvals) GetDuplicates(timeout time.Duration) []*structs.Evaluation {
	var timeoutTimer *time.Timer
	var timeoutCh <-chan time.Time
SCAN:
	b.l.Lock()
	if len(b.duplicates) != 0 {
		dups := b.duplicates
		b.duplicates = nil
		b.l.Unlock()
		return dups
	}
	b.l.Unlock()

	// Create the timer
	if timeoutTimer == nil && timeout != 0 {
		timeoutTimer = time.NewTimer(timeout)
		timeoutCh = timeoutTimer.C
		defer timeoutTimer.Stop()
	}

	select {
	case <-b.stopCh:
		return nil
	case <-timeoutCh:
		return nil
	case <-b.duplicateCh:
		goto SCAN
	}
}

// Flush is used to clear the state of blocked evaluations.
func (b *BlockedEvals) Flush() {
	b.l.Lock()
	defer b.l.Unlock()

	// Reset the blocked eval tracker.
	b.stats.TotalEscaped = 0
	b.stats.TotalBlocked = 0
	b.captured = make(map[string]*structs.Evaluation)
	b.escaped = make(map[string]*structs.Evaluation)
	b.jobs = make(map[string]struct{})
	b.duplicates = nil
	b.capacityChangeCh = make(chan string, unblockBuffer)
	b.stopCh = make(chan struct{})
	b.duplicateCh = make(chan struct{}, 1)
}

// Stats is used to query the state of the blocked eval tracker.
func (b *BlockedEvals) Stats() *BlockedStats {
	// Allocate a new stats struct
	stats := new(BlockedStats)

	b.l.RLock()
	defer b.l.RUnlock()

	// Copy all the stats
	stats.TotalEscaped = b.stats.TotalEscaped
	stats.TotalBlocked = b.stats.TotalBlocked
	return stats
}

// EmitStats is used to export metrics about the blocked eval tracker while enabled
func (b *BlockedEvals) EmitStats(period time.Duration, stopCh chan struct{}) {
	for {
		select {
		case <-time.After(period):
			stats := b.Stats()
			metrics.SetGauge([]string{"nomad", "blocked_evals", "total_blocked"}, float32(stats.TotalBlocked))
			metrics.SetGauge([]string{"nomad", "blocked_evals", "total_escaped"}, float32(stats.TotalEscaped))
		case <-stopCh:
			return
		}
	}
}